Zong Tian

Control Delay at Signalized Diamond Interchanges Considering Internal Queue Spillback

Control delay is the primary measure for determination of the level of service of signalized intersections. The existing analytical delay models usually work well for isolated intersections, but are not as effective when applied to diamond interchanges. The limited storage space between the two closely spaced intersections of a diamond interchange may cause queue spillback from the internal link to the outside roads. This property would give rise to unrealistic delay calculations for diamond interchanges with high traffic volumes. This paper describes the development of a new analytical delay model that takes into account the effects of internal queue spillback at diamond interchanges. Simulation studies are conducted to compare the effectiveness of the proposed approach with existing methods. The study shows that for low-overlap time conditions, the proposed model tends to agree with the Synchro and VisSim simulations and is better than Elefteriadous method, which tends to overpredict delay. For high overlaps, the Elefteriadou method, VisSim simulation, and the proposed model tend to agree, while Synchro diverges significantly by overestimating delay. The study contributes to the literature and practice by providing an open-source analytical model that can either be used as a stand-alone delay calculation model or as a supplement to the existing methods.

Control Delay Calculation at Diverging Diamond Interchanges

Diverging diamond interchange (DDI) is a form of diamond interchange attracting growing interest from traffic engineers and researchers. Conventional control delay calculation models are not effective when applied to DDI because of the possible internal queue spillback. This paper describes a method to calculate control delay at DDI using a new analytical model. The model was first developed for control delay calculations of external movements at conventional diamond interchanges. Through the addition of a function to calculate delay of internal movements, the model was successfully used at a DDI to calculate control delay of both internal movements and external movements. Simulation studies are also conducted to validate the new model. This study can be used either as a stand-alone delay calculation model or as a supplement to existing simulation methods. The model also shows promise for use in other signalized interchange configurations with two or more adjacent intersections.

Vehicle delay estimation at unsignalised pedestrian crosswalks with probabilistic yielding behaviour

Unsignalised pedestrian crosswalks are commonly adopted in school and residential areas. To enhance pedestrian safety, various types of signs and crosswalk markings have been implemented, which results in motorists’ probabilistic yielding behaviour and interrupted traffic flow patterns. Predicting the vehicular delay is of central importance to evaluate the level of service. However, as the interaction involves two random streams and is governed by the uncertain yielding behaviour, the analysis could be fairly challenging. In this paper, a novel method is proposed to estimate vehicular delay, which decomposes the vehicular stream into free-flow and queuing traffic. By explicitly considering the relation between the vehicular headway and the critical gap, the probability of a yielding event is derived to the expected proportion of queue formation, queue dispersion and free-flow periods. Equations of the average vehicular delay are given as a function of the vehicle volume, pedestrian volume and the yielding rate. The validation experiment using a stochastic simulation indicates that the proposed method consistently gives close estimations with absolute error less than 1 s.

Capacity of Multilane All-Way Stop-Controlled Intersections Based on the Conflict Technique

This paper presents an improved model for assessing the capacities of vehicular movements at all-way stop-controlled (AWSC) intersections with multilane approaches. In contrast to the current approach-based model incorporated in the 2000 edition of the Highway Capacity Manual (HCM), the proposed model simplifies the process of estimating the capacities of AWSC intersections and could be applied at any multilane AWSC intersections. On the basis of the conflict technique, a model was developed for estimating the capacities of vehicular movements at multilane AWSC intersections under three traffic demand levels: under-saturated, partially saturated, and fully saturated. Model parameters were calibrated by field data collected from 41 AWSC intersections with various lane configurations throughout the United States. The effectiveness and reliability of the proposed model were verified in comparison with the methodology of HCM 2000 with various lane configurations and traffic demands. The proposed model addresses the major limitations of the current HCM 2000 methodology, which deals only with the conditions of two lanes on each approach at AWSC intersections.

Feasibility of Using a Constant Acceleration Rate for Freeway Entrance Ramp Acceleration Lane Length Design

AbstractWhen estimating the required acceleration length of vehicles accelerating from a stopped position, a constant acceleration rate is sometimes assumed for the sake of simplicity. Nevertheless...

Recommendations for Acceleration Lane Length for Metered On-Ramps

AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) is currently used by most state departments of transportation in the United States in determining the design length of acceleration lanes of metered on-ramps; however, the recommended acceleration lengths have not been updated for several decades. This study aimed to develop a method for determining acceleration lengths at metered on-ramps. Vehicle location versus time information was collected via parallel cameras at seven metered on-ramps in California; then, a piecewise constant acceleration model was proposed to calculate the spot speeds of individual samples at predetermined locations. The percentile distance-versus-speed profiles at each ramp were built, and regression models were generated to predict the required acceleration length at a given merge speed. The 85th percentile data were recommended as the minimum acceleration length to accommodate most drivers in accelerating to a safe merging speed. The new recommendation was compared with the existing guidance in the Green Book. On the basis of 1,658 individual samples, it was found that the recommended acceleration lengths were shorter than the Green Book guidelines by 10% to 35%. Also, results showed that acceleration lengths for tractor trailer trucks were approximately 60% greater than the Green Book guidelines.

Impacts of traffic flow arrival pattern on the necessary queue storage space at metered on-ramps

ABSTRACT This paper compares queue lengths at the two different on-ramp configurations: arterial-to-freeway ramp and freeway-to-freeway connector. Mesoscopic queue length simulation models are developed based on the input-output method. The input-output method aims to model the traffic flow arrival profile; then, the mesoscopic simulation model generates random traffic volume to simulate the stochasticity of traffic flow. The simulation models are validated using field-collected data. Simulation results indicated that the traffic flow arrival pattern has significant impacts on ramp queue length; vehicle platoons released from the upstream signalized intersection tend to exacerbate ramp queue length. At metered ramps, the required queue storage length is found to be 5.7% of demand when demand is less than 500 vehicles per hour per lane (vphpl), or 3.9% when demand is greater than 500 vehicles per hour per lane (vphpl). In comparison, at metered connectors, the percent numbers are 4.7 and 2.1%, respectively.

Modeling the Impacts of Traffic Flow Arrival Profiles on Ramp Metering Queues

An adequate queue storage length is critical for a metered on-ramp to prevent ramp queue spillback to the upstream signalized intersection. Previous research on queue length estimation or queue storage length design at metered ramps has not taken into account the potential impact of various on-ramp traffic flow arrival profiles on ramp queue lengths. This paper depicts the traffic flow arrival profiles and queue generation processes at three different metered ramp categories. Based on a large number of microscopic simulation runs, it is found that, under a given demand-to-capacity scenario, the queue at a metered ramp with two on-ramp feeding movements is more likely to be cleared in a cycle than at a metered ramp with three on-ramp feeding movements. Also, the platoon dispersion effect significantly reduces the ramp queue length, and hence the queue storage needs at a metered ramp. In addition, this paper reveals that ramp queue length tends to increase linearly with upstream signal cycle length. The design of queue storage length for a metered on-ramp hence needs to fully consider the various ramp configurations and upstream signal timing settings.

Queue length estimation for a metered on-ramp using mesoscopic simulation

Abstract This paper proposed an analytical method for queue length modeling at a metered on-ramp, and developed a mesoscopic simulation model for queue length estimation under various demand-to-capacity scenarios. Queue length data were collected at four representative ramp metering locations for model validation; results showed that the queue length modeling method could properly capture the realistic queue profile, and the estimated queue lengths were close to the field observations. It was found that for under-saturated scenarios, queue length showed an exponential increasing trend with demand-to-capacity ratio; while for over-saturated scenarios, the queue length tended to increase linearly with demand-to-capacity ratio. Simulation results indicated that for under-saturated conditions, the required queue storage length was approximately 5.7 percent of on-ramp demand when demand was less than 500 vphpl, or 3.9 percent when demand was between 500 and 900 vphpl.

Right-Turn Traffic Volume Adjustment in Traffic Signal Warrant Analysis

An intersection with heavy right-turn volumes, without any reduction, might mislead a signal warrant analysis, and a different conclusion may be reached on whether a signal is warranted. Previous right-turn volume reduction methods based mostly on engineering judgment lack specific theoretical backgrounds. Therefore, this paper proposes a new method with theoretical justification. The method focuses on traffic operation principles centered on finding the delay equivalent relationship between right-turn and through traffic, that is, to equate the right-turn volume to through vehicles to produce the same control delay on a minor street. Equivalent factor tables were constructed on the basis of four geometric configurations. Combining with the various traffic volume distributions, more than 12,000 scenarios were analyzed. The volume ratio in the two directions of the main street was especially considered. The analysis showed that the uneven volume distribution in the main street had a greater effect on the minor-street right-turn movement. To use the equivalent factors efficiently, statistical regression models were developed. Last, the proposed method was applied to one signal warrant case and demonstrated promising results for its practical application in traffic signal warrant analysis.